Ceramic multi-layer component and method for the production thereof

ABSTRACT

Methods for producing a multilayer ceramic component with alternating ceramic layers and internal electrode layers include producing the ceramic layers using a ceramic mass. The methods also include producing the internal electrode layers using a metal paste that contains a portion of a chemically active additive; wherein the chemically active additive reacts chemically with at least one environmental component other than a metal portion of the metal paste.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/415,371, filed Mar. 31, 2009, now U.S. Pat. No. 8,776,364 which is adivisional of U.S. application Ser. No. 10/574,192, filed Mar. 28, 2006,now U.S. Pat. No. 7,525,241 which is a national phase application filedunder 35 U.S.C. 371 of International Application No. PCT/DE04/002167,filed Sep. 29, 2004, which claims benefit of German Application No.10345500.0, filed Sep. 30, 2003. The disclosure of the priorapplications are considered part of and are incorporated by reference inthe disclosure of this application.

TECHNICAL FIELD

This application relates to a multilayer ceramic component and a methodfor producing the component. The component includes a stack ofalternating ceramic layers and copper-containing electrode layers, whichserve as internal electrodes. The internal electrodes are connected toexternal contacts that are arranged on opposite, exterior sides of thestack, perpendicular to the multilayer structure. Internal electrodesthat are connected to different external contacts are in interlockingengagement with one another.

BACKGROUND

A component of foregoing type and a method for its production are known,for example, from DE 20023051.4.

DE 9700463 describes a method for producing green films for multilayerpiezoceramic components with Ag/Pd internal, in which a PZT-typepiezoceramic powder (PZT=lead zirconate titanate) is used.

The material and/or the process used to fire on the external electrodesthat contact the internal electrodes should, in principle, be selectedsuch that the electrode metal will not oxidize and the ceramic will notbe reduced. For this reason, a precious metal or a precious metal alloyis customarily used as the electrode material.

DE 19945933 describes an exemplary method for producing externalelectrodes in piezoceramic components comprised of PZT ceramic and Ag/Pdinternal electrodes. In this method, contacting of the Ag/Pd internalelectrodes is accomplished using a metal paste that has a silver contentof >65% and an organic binder, which is fired on at approximately 700°C. The firing on of the metal paste is conducted in an air atmosphere,because aromatic compounds contained in the organic paste binder cannotbe completely degraded under reductive conditions. However, this methodis not suited to a multilayer component with PZT ceramic andcopper-containing internal electrodes, since at customary debinderingand/or firing-on temperatures, reduction of the PZT ceramic andoxidation of the metallic copper are prevented under only one specific,very low oxygen partial pressure of <10⁻² Pa. Thus, the Ag/Pd externalelectrodes cannot be used in a multilayer ceramic component withcopper-containing internal electrodes.

DE 20023051 U1 describes a component that is less costly than multilayerpiezoceramic components that are based on a PZT ceramic and Ag/Pdinternal electrodes. This component uses copper-containing internalelectrodes in place of the expensive Ag/Pd internal electrodes. However,DE 20023051 U1 does not indicate which material would be suitable forfiring on external electrodes in a piezoceramic component withcopper-containing internal electrodes.

SUMMARY

Described herein is a multilayer ceramic component with internalelectrodes and external contacts for contacting the internal electrodes,in which the bonding strength of the external contacts is sufficient.Also described is a method for producing the component, in which theceramic is not reduced and in which neither internal nor externalelectrodes will oxidize.

The ceramic component of this application contains a stack ofalternating ceramic layers and copper-containing electrode layers thatserve as internal electrodes. The internal electrodes are connected toexternal contacts. The external contacts are arranged on exteriorsurfaces of the stack, opposite one another and perpendicular to themultilayer structure. The internal electrodes that are connected todifferent external contacts are in interlocking engagement with oneanother. The external contacts contain metallic copper. The bondingstrength of the external contacts to the stack is at least 50 N.

The ceramic layers may be comprised of ceramic green films that containa thermohydrolytically degradable binding agent and can comprise aferroelectric perovskite ceramic of the general composition ABO₃, suchas one of the PZT type Pb(Zr_(x)Ti_(1-x))O₃.

The application further describes a method for producing a ceramiccomponent, in which binder removal, or debindering, is performed and/orcompleted at a temperature of ≦300° C. The debindering takes place undera nitrogen stream with the addition of water vapor. The water vaporpartial pressure is set such that the corresponding oxygen partialpressure at the given temperature is between the equilibrium points forCu/Cu₂O and PbTiO₃/Pb. The equilibrium point corresponds to an oxygenpartial pressure at which both a reduced metal and a metal compound thatcorresponds to this metal are thermodynamically stable and can coexistwithout diffusing into one another.

Paste binders are completely separated out in a reduced atmosphere of,for example, <10⁻² Pa at a comparatively low temperature of ≦300° C.,because at higher debindering temperatures, the oxygen, which isinsufficient to burn off the carbon contained in the organic binder, ispartially drawn out of the ceramic lattice structure, which impairsproperties of the ceramic layers.

Complete separation of organic constituents occurs because debinderingis performed in a nitrogen stream that is charged with water vapor,resulting in hydrolytic separation. The addition of water vapor causesthe oxygen partial pressure to decrease thermodynamically. However, theoxygen partial pressure does not drop below a specific level at whichthe ceramic would begin to reductively degrade.

On the other hand, the oxygen partial pressure also will not exceed alevel at which metallic copper would begin to oxidize at the giventemperature. Thus the oxygen partial pressure is selected to be lowenough so that the reduction processes in the ceramic is adequatelyinhibited, while at the same time the copper contained in the metalpaste will not oxidize.

The oxygen partial pressure is maintained, or correspondingly adjusted,at all times to correspond to the temperature. This occurs not onlyduring the debindering process, but also during firing on of the metalpaste, such that in a p(T) diagram it lies between the equilibriumpoints for Cu/Cu₂O and Pb/PbO at each process temperature.

The proportion of copper in the metal paste may be >70%. Acrylic resinbinders may be used as the organic paste binders.

The foregoing makes it possible to use a (copper-containing) metal pastewith an organic paste binder to produce external contacts in apiezoceramic component with internal electrodes that contain copper.

To produce the external contacts, a copper-containing metal paste havinga copper content of >70 m %, for example 78 m %, a glass flow, and anorganic binder, for example acrylic resin binder, may be used.

Glass flow (glass frit) may be comprised essentially of PbO and SiO₂,but may also contain other components, such as Na₂O, Al₂O₃ and BaO. Theproportion of glass flow in the metal paste may be less than 5 m %. Thecomposition and the proportion of the glass flow are selected such thatthe glass frit contained in the metal paste of the external contactspartially diffuses into the ceramic, thereby increasing the bondingstrength of the external contacts to the sides of the stack.

First, using a known process produces a stack comprised of layers ofceramic material and electrode layers placed one on top of another,which in a finished component correspond to the internal electrodes. Theelectrode layers are comprised of a copper-containing metal paste andcan be applied to the layers made of a ceramic material, for example,via screen printing.

The copper-containing metal paste is also applied, for example, viascreen printing, on opposite sides of the stack of internal electrodesand ceramic layers, which are positioned one on top of another.

The binder is removed from the metal paste in a moist nitrogenatmosphere in a gas-tight furnace at a temperature of ≦300° C., afterwhich it is sintered at a higher temperature.

The copper-containing metal paste may be fired on at between 700 and860° C.

To prevent the reduction of the lead oxide contained in the ceramic,lattice bases made of metallic copper, which serve simultaneously asgettering material, can be used in production of a component describedherein.

Below, the component, and method for its production, are described ingreater detail with reference to exemplary embodiments and theassociated drawings. The figures contain schematic, not-to-scalerepresentations of various exemplary embodiments. Identical orequivalent components are assigned the same reference figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the structural design of a ceramicmultilayer component.

FIG. 2 is a half-logarithmic representation of oxygen partial pressure,which can be adjusted via the addition of water vapor, as a function ofthe temperature and the equilibrium curves for Cu/Cu₂O and Pb/PbTiO₃.

DETAILED DESCRIPTION

FIG. 1 shows a ceramic component with copper-containing externalcontacts AK1 and AK2. First copper-containing internal electrodes IE1are connected to the first external contact AK1. Secondcopper-containing internal electrodes IE2 are connected to the secondexternal contact AK2. The internal electrodes are separated from oneanother by ceramic layers KS.

The ceramic layers KS may possess piezoelectric properties and may beproduced using the PZT-type ceramic.

The component of FIG. 1 is a piezoelectric actuator. The electrodelayers and ceramic layers stacked one on top of another are referred toherein as a piezo stack.

The fired-on external contacts AK1, AK2 may be between 10 and 20 μmthick, e.g., 15 μm, thick. However, a different thickness for theexternal contacts may also be used.

The external contacts and/or internal electrodes may contain a certainproportion of ceramic, which may be less than 50 m % (m %—percentage bymass), and which in one embodiment is between 10 and 50 m %, e.g., 40 m%. In this embodiment, the ceramic proportion comprises ceramicparticles having a specific grain size, for example, an average grainsize of between 0.2 and 0.6 μm.

The ceramic portion in the metal paste serves to inhibit the formationof cracks and separation of the external contact from the piezo stack,which can result from different expansion properties of the ceramicmaterial and metallic copper.

The thermal expansion coefficient of PZT-type ceramic within thetemperature range extending from room temperature to the Curietemperature is approximately 1.5-2.0 ppm/K. In the same temperaturerange, metallic copper has a substantially higher thermal expansioncoefficient of approximately 19 ppm/K. By admixing the ceramic materialinto the metal paste, the expansion characteristics of the externalcontact are adjusted to the expansion behavior of the ceramic stack bothfor the processing of the component and for its later applicationswithin the specified temperature range of, for example, −50° C. to +150°C., wherein, for example, the application of an electrical field causesa deformation of the component.

To adjust the thermal expansion coefficient, a glass frit that containsa high proportion of SiO₂, for example 39 m %, may be used, since SiO₂has a high affinity for PZT-type ceramic.

In preparation of the copper-containing metal paste, first a ceramicpowder having an average grain size of, for example, 0.4 μm is dispersedin a solvent. The ceramic powder dispersion is then stirred into thecopper-containing metal paste of the above-named composition, and ishomogenized using a three-roller drawing machine. The viscosity of themetal paste may be between 10 and 20 Pas. Once the finished metal pastehas been applied to the side surfaces of the piezo stack, the paste isdried at approximately 80 to 140° C. in an air atmosphere. This isfollowed by debindering and sintering under the foregoing specifiedconditions, in which oxidation of metallic copper and reduction of PbOor PbTiO₃ are prevented. In this embodiment, selection of thedebindering temperature and the duration of the debindering processensures that, during debindering, both the binder components and theresidual solvent will be completely burned out of the metal paste.

Because the glass frit contained in the metal paste can diffuse quiteextensively into the ceramic, leaving behind hollow cavities in thesintered ceramic layers, the sinter temperature is selected to be lowenough (for example, 765° C.) that the glass additive can penetrate onlyin the area of the internal electrodes. Microscopic studies have shownthat at such a sinter temperature glass portions, above all siliconoxide, can be detected within a narrow area of the ceramic layers thatis adjacent to the external contacts. The external contacts adherefirmly and can be detached from the piezo stack with the application ofa high level of force, greater than 50 N. With the forceful detachmentof the external contacts, portions of the ceramic material are brokenoff, which indicates a high level of bonding strength for the externalcontacts on the piezo stack.

The ceramic proportion may be 40 m % relative to the solids content ofthe metal paste. This metal paste can also be used for internalelectrode layers.

A chemically active ceramic powder (or even some other chemically activeadditive), hereinafter also referred to as a ceramic additive, may bepresent in the electrode metallization, which under certaincircumstances can react chemically with the electrode metal, the organicbinder, the ceramic, and/or a reaction product of the latter, or canchemically bind certain components. Furthermore, the ceramic additivecan react with the process atmosphere, for example, it can releaseoxygen into the process atmosphere or can absorb oxygen so that theoxygen partial pressure is stabilized at least locally or temporarily.With a stable oxygen partial pressure, it is possible to protectinternal electrode layers or external contacts from oxidation and toprotect ceramic layers from reduction. The ceramic powder additiveensures that a metal oxide that has been formed as a result of processinstabilities in the electrode metallization is bonded, thus preventingan undesirable diffusion of this metal oxide into the ceramic layers.

The process of using a chemically inert ceramic powder in a metal paste,for example, to retard the sintering of the metal, is known in the art.In this application, however, the ceramic powder is used as a functionaladditive that is chemically active and can chemically react with itsenvironment. The chemical activity can be focused, for example, onbinding of Pb, which is released from the lead-containing ceramicmaterial during sintering. It is also possible for the chemically activeceramic powder to bind another component from the ceramic massespecially during sintering, or to promote the release of certaincomponents, such as oxygen from the ceramic mass or from the bindercontained in the ceramic mass or in the metal paste. The ceramic powdershould not chemically react with the metal portion of the metal paste.

In contrast to the metal paste that is used for the external electrodes,in the metal paste that is suitable for use in the internal electrodelayers, preferably no glass additives are used. (Zr, Ti)O₂ isparticularly well suited for use as the chemically active ceramicpowder. In place of a chemically active ceramic powder, the metal pastecan also contain another chemically active additive, or in addition tothe chemically active ceramic powder it may contain portions of othersubstances, for example BaO₂ and/or MgO.

The addition of a ceramic powder in the case of the internal electrodesfurther serves to improve adhesion between the internal electrodes andthe ceramic layers that surround them. A fine distribution of theceramic particles among the metal particles especially serves to preventsinter neck formation. The sinter necks represent a localizeddisconnection of the internal electrodes, in which the metal coatingbecomes detached from the ceramic layer and/or becomes pulledback—especially around the edges—so that the internal electrodes assumea net-like structure, which is not replicable from component tocomponent. The method of achieving a homogeneous internal electrodestructure by adding a precious metal or a precious metal alloy is knownin the art. However, the addition of the ceramic powder as describedherein also provides a substantial cost advantage over the known method.

FIG. 2 shows a half-logarithmic representation of an oxygen partialpressure p₀₂ that can be adjusted based upon the temperature—by addingwater vapor—and is illustrated by way of example by the curve 3, thenumerically calculated equilibrium curve 1 for Cu and Cu₂O, and thenumerically calculated equilibrium curve 2 for Pb and PbTiO₃.

The equilibrium curve 1 indicates the partial pressure of O₂ at theselected temperature, at which metallic Cu and Cu₂O can coexist.Metallic copper exists only at an oxygen partial pressure p₀₂ that doesnot exceed the equilibrium level, i.e. below the equilibrium curve 1.Because only Cu₂O is stable above the curve 1, at an oxygen partialpressure that exceeds the equilibrium value at the selected temperaturean undesirable oxidation of the metallic copper will occur.

The equilibrium curve 2 indicates the partial pressure for O₂ at theselected temperature, at which metallic Pb and PbTiO₃ can coexist.PbTiO₃ exists only above the curve 2. At an oxygen partial pressure p₀₂that drops below the equilibrium level at the selected temperature, thePbTiO₃ contained in the ceramic will be reduced to Pb.

For this reason, at least during the debindering phase of a process forproducing the above-described multilayer ceramic component, the oxygenpartial pressure p₀₂ is adjusted by adding water vapor such that theoxygen partial pressure does not exceed the maximum level p_(max) asdefined by the curve 1, at which metallic copper is still stable, butalso will not drop below the minimum level p_(min) as indicated by thecurve 2, at which lead titanate is not yet reduced, i.e.,p_(min)<p₀₂<p_(max) at the given debindering temperature T_(E). Thepermissible range for adjustment of the oxygen partial pressure isbetween the curves 1 and 2.

Curve 3 shows an optimal oxygen partial pressure to be established basedupon temperature and within a moist atmosphere. The quantity of watervapor to be added can be calculated in principle from the curve 3. It isalso possible to manually or automatically control the drop in theoxygen partial pressure by adding water vapor, in such a way that thepreset threshold values are not exceeded.

What is claimed is:
 1. A method for producing a multilayer ceramiccomponent with alternating ceramic layers and internal electrode layers,comprising: producing the ceramic layers using a lead-containing ceramicmass; producing the internal electrode layers using a metal paste thatcontains a portion of a chemically active ceramic powder; positioningthe internal electrode layers and ceramic layers one on top of another;and connecting the internal electrode layers to external contacts,wherein the chemically active ceramic powder comprises (Zr, Ti)O₂ anddoes not chemically react with a metal component of the metal paste, andwherein Pb is bonded as a result of a chemical reaction between the (Zr,Ti)O₂ and at least one component of surroundings of the Pb.
 2. Themethod of claim 1, wherein the at least one component of thesurroundings of the Pb comprises oxygen, at least one component of theceramic mass, and a binder or solvent that is contained in the metalpaste or the ceramic mass.
 3. The method of claim 1, wherein as a resultof a chemical reaction between the chemically active additive and anenvironment, oxygen is released and/or Pb and/or Cu are bonded.
 4. Themethod of claim 1, wherein the metal paste contains a non-preciousmetal.
 5. The method of claim 4, wherein the metal paste contains Cu orNi.